One of the striking properties of neuronal organization in many parts of the brain, as first described in somatosensory and visual cortices (Mountcastle, 1978; Huben and Wiesel, 1977), is that neurons with similar receptive field properties are grouped together into what, in neocortex, are columnar ensembles of neurons. Elucidating the developmental mechanisms responsible for this aspect of neuronal organization will very likely shed light on some of the most fundamental rules governing the formation of neural circuits in this part of the brain. This, in turn, will ultimately simplify one of the challenging problems in contemporary cortical neurobiology, namely, uncovering the basic rules of cortical information processing and defining the neuronal circuits that implement these computations. One aspect of cortical development that has recently come into focus is the presence of widespread coupling between neighboring neurons (Yuste et al., 1992; Peinado et al., 1993a,b). The investigator has speculated that this phenomenon may be involved in shaping the organization of neocortex by enhancing near-neighbor interactions during a critical period in cortical development when new synapses are being formed at a fast pace. The work proposed in this application seeks to combine state-of-the-art electrophysiological and imaging techniques to characterize the nature and significance of gap junctions formed between neurons in the developing rat cerebral cortex. Specifically, fluorescent gap junction tracers will be used in combination with low light-level, high signal-to-noise imaging techniques and whole cell recording. Patch electrodes will be used to study gap junction coupling in the cortical brain slice preparation where the neuronal environment and connectivity is representative of the situation in the intact brain. This approach will be applied to the developing neocortex to: (a) develop the means for visualizing sets of coupled neurons in living brain slices; (b) measure dye- and electrical-coupling under normal conditions and following exposure to putative gap junction modulating agents; and (c) record patterns of neuronal activity in sets of coupled neurons using both electrophysiological and imaging techniques. The proposed studies will increase our understanding of the function of neuronal gap junctions in cortical development and in the development of other brain regions, as well as lay the groundwork for the study of other types of multicellular neuronal interactions requiring simultaneous anatomical and electrophysiological approaches both in developing and mature brain. A strong motivating factor in these studies is the conviction that further progress in our understanding of how brain circuits operate will be intimately tied to our ability to study the behavior of groups of neurons with means that preserve the fine spatial and temporal resolution typical of single-cell approaches.